(a) Technical Field
The present invention relates to a polyurethane foam with improved water resistance for use in an automobile steering wheel.
(b) Background Art
Polyurethane resin is formed by a chemical reaction between a liquid-phase polyol and a liquid-phase isocyanate, which include intramolecular hydroxy functional group (—OH) and intramolecular isocyanate functional group (—NCO), respectively. A polyol is referred to as monol, diol, triol, etc., depending on the number of intramolecular hydroxy functional group. An isocyanate may also be divided into monoisocyanate, diisocyanate, etc., depending on the number of intramolecular functional group.
Polyurethane resin is generally prepared by reacting a polyol having two or more functional groups with an isocyanate having two or more functional groups. Urethane group is formed by a chemical reaction between the two functional groups existing at the end of each molecule as shown in Scheme 1 below.R—NCO 30 R′—OH→R—NH—COO—R′  (Scheme 1)
A resin including intramolecular urethane group is referred to as polyurethane resin.
Water reacts with isocyanate molecule in the formation of urethane, thereby forming unstable carbamic acid, which is decomposed to produce amine and carbon dioxide (Scheme 2).
Thus produced amine reacts with isocyanate to form urea group (—NHCONH—) as shown in Scheme 3. Carbon dioxide gas forms small foams in polyurethane resin, thereby forming a dispersed cell structure in polyurethane.R—NCO+H2O→R—NH—COOH→R—NH2+CO2   (Scheme 2)R—NH2+R′—NCO→R—NH—CO—NH—R′  (Scheme 3)
Polyurethane foam is used for preparing automobile parts due to its superior properties such as low density, high mechanical property and high heat resistance. In particular, it is widely used as integral skin foam to obtain a leather-like appearance for a steering wheel.
Polyurethane may be divided into an expanded polyurethane and a non-expanded polyurethane, and it is generally the expanded one that is used for automobile parts. For expansion, a foaming agent is included in polyol before a forming reaction. The foaming agent may be divided into physical foaming agent and chemical foaming agent. The physical foaming agent is liquid-phase chemical compound with low boiling point such as fluorocarbon-based compounds (CFC, HCFC, etc.) and pentane-based compounds (pentane, cyclopentane, etc.), which causes expansion when boiled at a relatively high temperature. However, Montreal Protocol or other international regulations calls for an end to the use of the fluorocarbon-based compound to prevent global warming and protect an ozone layer. The pentane-based compounds are difficult to handle due to inflammability and explosiveness. The fluorocarbon-based compounds and the pentane-based compounds may cause problems related to safety and user's health.
Recently, water blown products are being extensively studied to solve these problems by using chemical foaming agent such as water, and some of them have been commercialized.
However, the conventional waterborne polyurethane foam needs to be improved in terms of durability as shown in FIG. 1. There are various issues to be resolved, for example extension of the term of guarantee, aggravated working conditions due to global warming, accelerated aging under various operation conditions as automobile market area is globalized, hydrolysis due to humidity, etc. To solve these problems, in-mold paint has been used so far to minimize cracks and delamination due to abrasion and exposure to moisture or sweat. However, polyurethane may easily be hydrolyzed when the in-mold paint layer is damaged under severe operation conditions, thus necessitating polyurethane to improve durability.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.